Nude mice congenic to several inbred mouse strains will be produced. These mice will be maintained in germ-free and specific pathogen-free colonies. They will be used for an analysis of B-cell development. The relationships between lymphocytes capable of responding to certain thymus independent antigens and those capable of responding to thymus dependent antigens will be investigated. The role of antigen-antibody complexes in this relationship will be studied. In particular, the idea that there is immunoglobulin isotype specific regulation of the immune response will be tested.